<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <input type="text"  class=" input" value="0" onclick="">
    <input type="button"  class="add" value="+1" onclick="Add()">
    <input type="button"   class="min" value="-1" >


</body>
<script>
    function Add(){
        let a = document.querySelector('.input')
        a.value=parseInt(a.value)+1;
      
    }
    let b=document.querySelector('.min')
    let a = document.querySelector('.input')
    b.onclick=function(){
        a.value=parseInt(a.value)-1;
      
    }
</script>
</html>